Firing ranges for low fire clay

I usually work in terra cotta or other low fire clays. When there is a firing range given for a clay, I understand that the high end of the range will provide the most strength in the finished, fired piece (although I'm told that it won't become vitreous if it's lowfire clay), but what, specifically, is the lower number telling me? Will there be a significant difference in the strength of the piece if I fire it at the high end of the range or the low end? And is a low fire clay a poor choice for a necklace pendant? Will it be fragile no matter how high it's fired?